THURSDAY, June 11, 2015 (HealthDay News) -- A study of more than 67,000 women suggests that those who are obese and postmenopausal may face significantly higher odds for breast cancer compared with slimmer women.

The risk for breast cancer among the most obese women in the study was up to 58 percent higher than for normal-weight women.

In this study, the researchers said the most obese women had a body mass index (a measure of body fat) of 35 or higher. A woman who is 5-foot-7 and weighs 225 pounds would fall into that category.

"Obesity is a risk factor for breast cancer that is modifiable, making a healthy weight very important for prevention," said lead researcher Marian Neuhouser, a professor of epidemiology at the Fred Hutchinson Cancer Research Center in Seattle.

In addition to increasing the risk for breast cancer, obesity was associated with worse outcomes, she said. Moreover, women who gained weight in early postmenopause increased their risk even if they were a normal weight at the start of the study, Neuhouser said.

While the study found an association between obesity and an increased risk of breast cancer, it did not prove a cause-and-effect link.

Neuhouser explained that the increased risk for breast cancer is probably due to an increase in estrogen.

"Obesity is known to increase estrogens in the postmenopausal women because estrogen is made by fat tissue," Neuhouser said. "Fat tissue also secretes inflammatory factors and is associated with insulin resistance -- all of which may increase breast cancer risk."

Women who lost weight during the study reduced their risk for breast cancer, Neuhouser said. But the study was not about weight loss, therefore other studies are needed to really answer that question, she added.

"We can't change our genes or family history, but we can change our lifestyle habits and aim to maintain a healthy weight to lower breast cancer risk," Neuhouser said.

The report was published online June 11 in the journal JAMA Oncology.

For the study, Neuhouser and her colleagues collected data on more than 67,000 postmenopausal women who took part in a study called the Women's Health Initiative from 1993 to 1998. During an average 13 years of follow-up, more than 3,300 women developed breast cancer.

The researchers found that very obese women were at risk for estrogen- and progesterone-driven breast cancer, but not for other types. These women were also more likely to have large tumors and cancer that spread beyond the breast and into the lymph nodes.

Women who gained more than 5 percent of their body weight over the years of the study also had an increased risk for breast cancer, according to the researchers.

Among women who were already overweight or obese, Neuhouser's team found no change in risk for breast cancer whether they lost or gained weight over the 13 years of follow-up.

In addition, hormone replacement therapy had no effect on the risk for breast cancer, regardless of weight, the researchers said.

Dr. Clifford Hudis, chief of breast medicine service at Memorial Sloan Kettering Cancer Center in New York City and coauthor of an accompanying journal editorial, said, "Obesity is a growing concern in many ways, including its contribution to cancer, and we need to prioritize scientific and public policy decision-making to help limit the health burden it brings."

He added that knowing the role obesity plays in the increased risk for cancer may help in understanding how and why some groups of people develop cancer, he said.

"From that, we may be able to develop better prevention and treatment in the future," Hudis said.